This invention relates generally to the field of photoresists. In particular, this invention relates to the field of photoresists having improved stripping properties, especially suitable for use in printed wiring board manufacture.
Photoresists are photosensitive films used for transfer of images to a substrate. A coating layer of a photoresist is formed on a substrate and the photoresist layer is then exposed through a photomask to a source of activating radiation. The photomask has areas that are opaque to activating radiation and other areas that are transparent to activating radiation. Exposure to activating radiation provides a photoinduced chemical transformation of the photoresist coating to thereby transfer the pattern of the photomask to the photoresist-coated substrate. Following exposure, the photoresist is developed to provide a relief image that permits selective processing of a substrate.
A photoresist can be either positive-acting or negative-acting. For most negative-acting photoresists, those coating layer portions that are exposed to activating radiation polymerize or cross-link in a reaction between a photoactive compound and polymerizable agents of the photoresist composition. Consequently, the exposed coating portions are rendered less soluble in a developer solution than unexposed portions. For positive-acting photoresists, exposed portions are rendered more soluble in a developer solution while areas not exposed remain comparatively less developer soluble.
In general, photoresist compositions include at least a resin binder component, a monomer and a photoactive agent. A wide variety of polymeric or resin binders may be used in photoresists. Such polymeric binders may include, as polymerized components, one or more acid functional monomers such as acrylic acid or methacrylic acid. For example, U.S. Pat. No. 5,952,153 (Lundy et al.) discloses photoimageable compositions containing polymeric binders having sufficient acid functionality to render the photoimageable composition developable in alkaline aqueous solution. U.S. Pat. No. 4,537,855 (Ide) discloses polycarboxylic acids used to form polymerizable ester derivatives with ethylenically unsaturated compounds. Such polymerizable ester derivatives are used to form the polymeric binders for photoimageable compositions.
Monomers typically useful in photoresist compositions are any which are cross-linkable. Such monomers cross-link to form a polymerized network having a very large, i.e. infinite, molecular weight. The polymeric binders do not participate in such cross-linking. Rather, the monomers form a polymerized network around the polymeric binders. Typically, polymeric binders contain pendant groups, such as carboxylic acids that react with the developer to increase the water solubility of the binder. Thus, in the unexposed portion, the acid functional polymer is salted in the alkaline solution, while in the exposed area (protected by the cross-linked monomers), the polymer is not affected. During stripping, the polymerized network (of cross-linking monomer) is attacked or degraded by the stripper allowing it to be removed, whereas the polymeric binder remains relatively unaffected by such strippers.
Photoresists may be either liquid or dry film. Liquid photoresists are dispensed on a substrate and then cured. Dry film photoresists are typically laminated to a substrate. Such dry film photoresists are particularly suitable for use in printed wiring board manufacture. One problem with conventional dry film photoresist compositions is that they are difficult to strip from electrolytically plated circuit boards using conventional alkaline aqueous stripping solutions, e.g. 3% sodium hydroxide solution. This problem arises from the demand of circuit board manufacturers to reduce the size of printed circuit boards, while increasing their functional capabilities. Consequently, the circuit lines and spaces on the circuit boards have continued to shrink, as more circuitry needs to be accommodated in smaller spaces. At the same time, metal plating heights have also increased above the thickness of the photoresist. This causes the metal to hang over the photoresist, resulting in a very narrow space containing the photoresist being virtually encapsulated by the overplated metal. The photoresist then becomes trapped by the plated overhang, making it difficult to attack and strip by conventional methods. If the photoresist is not completely stripped or removed, ragged copper circuit lines will result after etching which are unsuitable as they can cause short circuiting of the board.
Some circuit board manufacturers have tried thicker photoresists to accommodate the increasing plating heights, however, this approach is more expensive and limits resolution of the circuit lines. Typically, organic-based (amine- or organic solvent-containing) alkaline stripping solutions are used which produce a smaller stripped particle to facilitate stripping. While such organic-based strippers remove the resist better, they are expensive relative to inorganic-based strippers (e.g. sodium or potassium hydroxide) and have more waste treatment and environmental concerns associated with them. Solvent-strippable photoresists are much less desirable due to workplace regulations limiting or reducing solvent emissions.
Certain polymer binders have been described optionally containing one or more multifunctional monomers. For example, U.S. Pat. No. 5,939,239 (Lundy et al.) discloses polymer binders containing acid functional monomers optionally copolymerized with another monomer, including certain multifunctional monomers. The multifunctional monomers disclosed are tri- or tetra-functional (meth)acrylates esters or relatively low molecular weight, i.e. typically ≦450, difunctional (meth)acrylates esters. Polymer binders containing such tri- and tetra-functional monomers or such relatively low molecular weight difunctional (meth)acrylate esters suffer from gel formation, which makes such polymers unsuitable for use in photoresist compositions.
It is thus desirable to provide photoresist compositions that are easily removed using alkaline aqueous inorganic-based stripping solutions, and that do not form gels.